In digital landline telephone systems, speech is digitized, encoded to a standard law used in public networks (in particular the A and μ laws), and conveyed by 64 kilo bits per second (kbps) circuits. In radio communications systems it is generally necessary to reduce this bit rate, in particular at the radio interface. To this end radio terminals incorporate a speech compression function.
In some systems, such as the Global System for Mobile communications (GSM), which is a second generation (2G) radio communications system, speech is transported in compressed form in the radio access network between base stations and a transcoder and rate adapter unit (TRAU) located between the radio access network and the core network, usually in 16 kbps pulse code modulation (PCM) channels, a PCM channel corresponding on the radio interface to one full rate speech channel or to two half-rate channels, for example. The TRAU transcodes speech between the 64 kbps coding law A and the full rate or half-rate code.
Other systems, such as the Universal Mobile Telecommunications System (UMTS), which is a third generation (3G) radio communications system, offer circuit mode communication with an end-to-end bit rate of 64 kbps. This caters in particular for videotelephone calls, which require a higher bit rate than telephone calls and therefore cannot be supported adequately by second generation systems.
Speech coding/decoding for videotelephone calls at an end-to-end bit rate of 64 kbps, for example, may be carried out in accordance with the H.324 standard, which is described in ITU-T Recommendation H.324, including appendix C thereof (“Multimedia telephone terminals over error prone channels”) and where appropriate appendix H thereof (“Mobile multilink operation”), which covers this kind of coding/decoding. Technical Specification TS 26.111, version 5.0.0, “Codec for Circuit-switched Multimedia Telephony Service; Modifications to H.324”, published in June 2002 by the 3rd Generation Partnership Project (3GPP), which is more specifically adapted to UMTS terminals, also covers this kind of coding/decoding.
Since it conveys voice and video simultaneously, a videotelephone call necessitates a high transmission bit rate, which the UMTS can offer. In contrast, the GSM generally proves unable to support videotelephone calls since the maximum bit rate authorized on its radio segment is too low for this purpose.
A problem arises in heterogeneous radio communications systems, for example a system comprising second generation (GSM) plant and third generation (UMTS) plant, because a terminal may initiate a videotelephone call when it is under the control of 3G plant, which call cannot continue if the terminal is transferred (handed over) to the control of 2G plant, for example as a result of the terminal moving to an area in which no 3G plant is available. The videotelephone call is then cut off, which is particularly frustrating for the user.